1. Field of the Invention
This invention relates to safe operation of wet oxidation systems using pure oxygen.
2. Description of the Prior Art
Wet oxidation is a well established process for treating aqueous waste waters or sludge which contain oxidizable substances; more than one hundred wet oxidation units are in commercial operation. A number of patents and other publications disclose wet oxidation processes in which air is used as the source of oxygen for oxidation of the wastes. A smaller number of patents disclose that pure oxygen or an oxygen enriched gas may be the oxidizing agent in wet oxidation processes. Among these are Schoeffel U.S. Pat. Nos. 3,042,489 and 3,097,988; and Pradt et al. U.S. Pat. No. 3,654,070. However, none of these patents disclose how a wet oxidation process using pure oxygen would be designed or operated.
In wet oxidation systems, aqueous and gaseous phases coexist at elevated pressures and temperatures. System pressures are chosen so that there will always be an aqueous phase. Oxidation reactions consume oxygen and generate carbon dioxide. When the aqueous phase has a neutral or low pH, a major portion of the carbon dioxide formed by wet oxidation will remain in the gaseous phase, diluting the oxygen. When the aqueous phase is caustic, however, much of the carbon dioxide will be absorbed in the aqueous phase.
The quantity of water vapor which is present in the gas phase is a function of temperature, pressure, and quantity of non-condensible gases (NCG), and can be determined by known thermodynamic relationships. For a given system operating at a nearly uniform pressure, the degree of gas dilution by water vapor is much greater at the higher temperatures.
In prior art processes using air as the source of oxygen, the percentage of oxygen in the gas phase at elevated temperatures and pressures is considerably less than 21 percent, even without any oxygen consumption. For example, at 550.degree. F. and 1000 psi pressure, water vapor dilutes the oxygen from its original 21 percent to a concentration of about 5 percent. As oxygen is consumed its concentration at reactor conditions drops to very low values. Therefore, pure oxygen or oxygen enriched gas can be used advantageously in enhancing the rate and completeness of oxidation, so long as the safety of the process can be ensured.
Gaseous oxygen, when diluted to a concentration of 21 mole percent as in the form of air, is safe to handle, even when compressed to quite high pressures.
However, oxygen at higher concentrations, especially high purity oxygen, is likely to undergo rapid, spontaneous combustion when placed in contact with organic or other oxidizable substances at pressures above atmospheric, even at room temperature. In the wet oxidation process, high concentrations of oxidizable materials are deliberately oxidized. It is vital to control the process so that transient excursions of temperature, pressure, and thermal efficiency are minimal and hazardous operating conditions do not occur.
Moreover, many metals such as steel, aluminum and titanium, for example, will burn vigorously in the presence of oxygen once an ignition has occurred. Titanium itself has been shown to be capable of undergoing spontaneous combustion under certain conditions in the presence of oxygen and water at elevated pressures.
In the handling of oxygen, traditional safety practice has emphasized selection of materials of construction which will not themselves undergo spontaneous combustion at design operating conditions, and strict cleanliness standards to ensure that no contaminants capable of spontaneous combustion are present in the system. In wet oxidations, however, the choice of materials of construction is nearly always constrained by the corrosive properties of the wastewater, sludge, or slurry being oxidized. Thus, titanium or titanium alloys may be dictated as the material of construction when severe corrosion of iron- or nickle-based alloys is indicated. Moreover, the wet oxidation system treats wastewaters, sludges, or slurries which may contain up to ten percent or even higher concentrations of organic substances, and the reactor interior surfaces may typically be contaminated with substances capable of spontaneous combustion upon contact with high oxygen concentrations at high pressures.
When pure oxygen is used in a wet oxidation system, purge water must be mixed with the oxygen in order to prevent backflow of waste or fuel into the oxygen supply lines. Such backflow may contaminate those lines with combustable substances and lead to unsafe conditions. Also, when pure oxygen is used in a wet oxidation system it must be diluted by an inert gas to prevent formation of unsafe oxygen concentrations in the reactor and down-stream piping. The diluting gas can be nitrogen, air, carbon dioxide, or another inert gas. If a first diluting gas is being injected into a wet oxidation system and if sufficient carbon dioxide is being generated and liberated into the gas phase to perform the dilution of the oxygen, then the flow of the first diluting gas may be shut off.
It has been discovered that when pure oxygen is used in wet oxidation systems, most of the opportunities for unsafe operation occur during startup, shut-down and transient process upsets during otherwise steady-state operation. Specific sequences of operating steps have now been found which are necessary to safe operation of such pure oxygen wet oxidation systems.
Some of the individual steps of this invention have been disclosed previously. However, the sum total of all the steps and procedures of this invention are unique and economically valuable.